| Gilgel Abay | |
| Map: | Lake tana and rivers.svg |
| Other Name: | Lesser Abay |
| Mouth Coordinates: | 11.7987°N 37.1253°W |
| Country: | Ethiopia |
| State Type: | Region |
| State: | Amhara |
| Region Type: | Zone |
| Region: | West Gojjam |
| Source1 Coordinates: | 11.0115°N 37.1523°W |
| Source1 Location: | Near Gish Abay |
| Mouth: | Lake Tana |
| Mouth Location: | SE of Kunzila |
| Progression: | Lake Tana → Blue Nile → Nile → Mediterranean Sea |
| River System: | Nile Basin |
| Etymology: | Literally "Lesser Nile" |
| Basin Population: | 1,220,000 |
The Gilgel Abay (Amharic: ግልገል አባይ, Gǝlgäl Abbay), or Lesser Abay, is a river of central Ethiopia. Rising in the mountains of Gojjam, it flows northward to empty into south-western Lake Tana in a bird's-foot delta. Tributaries of the Gilgel Abbay include the Ashar, Jamma, Kelti and the Koger. It was regarded as the true source of the Nile for a long time and the Jesuit priest Pedro Paez visited it in 1618. The name Gilgel Abbay means Lesser Nile, as Abbay is the name for the Blue Nile.
It is a meandering river, with a catchment area of . It is 71 meters wide near its mouth, with a slope gradient of 0.7 m/km. The average diameter of the bed material is 0.37 mm (sand).[1]
The river carries annually 22,185 tonnes of bedload and 7.6 million tonnes of suspended sediment to Lake Tana.[1]